Equipment that is electrically operated, or that incorporates moving structures containing electrically conductive materials or charged dielectrics, will generate static and time-varying electromagnetic fields during operation. These fields may be faint even in close proximity to the source, and will attenuate as the distance from the source is increased. Nevertheless, detectable components of these signals may exist at great distances from the source. Often great care is taken to design equipment, such as military equipment, to minimize the likelihood that unintended electromagnetic emissions will reveal the location of the equipment. Despite the care taken to reduce such emissions, low level electromagnetic signals may still exist at great distances and can be measured. Weak electromagnetic signals may also be utilized in numerous other applications, such as in communication systems, natural resource exploration, scientific research, meteorological monitoring, localization, and navigation.
Similarly, various bio-physical signals are generated by the human body. For example, ionic currents within neurons of the brain will generate voltage fluctuations and magnetic fields during synaptic transmission. Although these signals may be weak, they can be measured and used in various diagnostic applications. Conventionally, numerous highly sensitive magnetometers are employed during magnetocephalography to detect magnetic fields, and numerous electrodes are employed during electroencephalography to detect electrical activity.